Sunday, 20 February 2011

Today, there are two main groups of large, herbivorous hoofed mammals; one that includes those with cloven hooves, and one that includes the horses. But that hasn't always been the case.

Millions of years ago, South America was an island continent, as separated from the rest of the world as Australia is today. It separated from the other continents before either of the living groups of hoofed mammals had evolved, but the creatures that would give rise to those groups - a rather vaguely defined group of primitive herbivores called the condylarths - did exist, and were already present. In their isolation, these animals evolved into a whole array of large herbivores, creating a rich, and to modern eyes, rather strange-looking, mammalian fauna.

When North America finally hit its southern neighbour, it brought with it large mammalian carnivores, which had previously been absent. Every single one of these strange creatures has since died out - most of them not long after the arrival of sabretooths and jaguars, although at least one species probably survived long enough to be wiped out by humans. Yet, for millions of years, far longer than the paltry period of time since their disappearance, these were among the dominant mammals of their day.

There were many families, varying from each other at least as much as horses do from antelopes, but one of the more successful ones was that of the macraucheniids. A look at even this highly simplified version of their family tree reveals just how distant they were from living groups, for everything at all close to them is also extinct:

Macraucheniids Adianthids Prototheres

^ ^ ^

| | |

| | |

----------------- |

| | Notoungulates

| | etc.

------------------------ ^

| |

| |

-------------------------------

|

|

The macrauchaeniids looked much like llamas in their bodily proportions, which makes sense, as they lived in a similar environment. (The llamas themselves, of course, arrived only when North America did). Like the living hoofed mammals, they had a reduced number of toes. In their case, they had three, with the central one being the largest and bearing the most weight. This is somewhat similar to modern rhinoceroses, although the feet of most macrauchaeniids would have been more slender, closer to those three-toed animals from which horses evolved. On the other hand, while all the living species of large herbivore have lost many of their teeth, keeping only those best suited to chewing plants, some of the macrauchaeniids had the full set of 44, although these were clearly adapted for a herbivorous diet.

One of the more distinctive features of the group is the position of their nostrils, which are far up on their skull, not at the tip of the snout, where one would expect to find them. This position became more extreme as the family evolved, so that the bones that would normally form the top of the snout (and in humans form the bridge of the nose) are remarkably short. It has generally been assumed that this means they had a short, flexible trunk, running forward from the nostrils over the face. A very similar pattern is, after all, seen in the skulls of living tapirs, which do have a proboscis of exactly this sort.

Because such a trunk, not being made of bone, has never been found in any fossils, its hard to know for sure that it really existed. It seems likely, but there are at least some other possibilities. For example, nostrils placed far back on the head make it easier for an animal to swim without drowning, which would be useful if they lived in a particularly wet environment.

Although the oldest fossil macrauchaeniids go back further still, it seems that the family really began to take off and diversify between about 29 and 21 million years ago. Yet most of what we know about them comes from more recent specimens, typically less than about 20 million years old, and in particular, from Macrauchaenia itself, for which the group is named. Macrauchaenia is undoubtedly a fascinating animal, and, at around five feet high at the shoulder, one of the larger members of its family. But, having died out just 20,000 years ago, one thing it doesn't tell us is how the family came to be in the first place.

The earlier forms, from the little we know of them, were less specialised than their descendants later became. For example, the nostrils were in a less extreme position, which suggests that the trunk, if present, would have been very short, and resembling more of an enlarged nose than anything else. As is often the case, however, the fossils that we do have are frustratingly incomplete. Its actually quite unusual to have anything approaching a complete skeleton in any fossil except the most recent ones, and the macrauchaeniids are no exception.

In this context, last November an Argentinian fossil was described that, for the first time, includes both a complete skull and some other parts of the skeleton belonging to a macrauchaeniid that dates from the time that we think the family was really beginning to diversify. Skulls are good for identifying fossil mammals, since they tend to be much more distinctive than other bits of the skeleton, although at least one early macrauchaeniid species has been described based on the discovery of part of a foot and the associated ankle. (This may not sound like much, but its frequently all there is to work with, and, at least in this case, the peculiar three-toed feet are fairly identifiable).

This new fossil belongs to a species that's already known, Cramauchenia normalis, although it hadn't been previously known to have existed so early. When I say that there are some other parts of the skeleton that can be matched with the skull, its still not a lot - the right humerus, a small bit of the left foot, and one toe. But that's actually quite a lot more than you often get, and its worth noting that when the species was originally described by Florentino Ameghino back in 1902, all he had to work on was half a crushed skull.

This skull, however, is relatively well preserved. We can tell, for example, that there is nowhere for sizeable muscles to attach to the area around the nostrils. Which means that, whatever the case might be for later species, this one probably didn't have a trunk. The shape of the skull and teeth also tell us that the animal was a browser, feeding on relatively soft leaves and shrubs, rather than tough grasses. While we may not have a complete limb, the shape of the bones we do have indicates that the animal would probably have been good at running, enabling it to escape quickly from predators.

What predators, you might wonder, given that I've already mentioned this animal lived long before the arrival of large mammalian carnivores. Its true that it would never have had to face large cats, wolves, bears, or any of the other creatures that we normally think of as preying on, for example, deer or antelopes - and, with a skull 27cm long, that's roughly the size range we're thinking of. But, strange as some of the mammals of this time might have been, the dominant South American predators were perhaps stranger still.

For it was not lions or sabretooths that Cramauchenia would have needed to flee from, but giant, flightless, terror birds. South America has changed a lot since those days.

Sunday, 13 February 2011

In many animal species, there are more females born than males, or vice versa. Indeed, at the extremes, there are species that have done away with males altogether. Things don't tend to be quite so stark when we get to mammals, but its still the case that one sex can be noticeably more common than the other. With adults, it could be the case that one sex dies off more quickly than the other, leaving the other sex more common in the population. But the fact is, we see this uneven distribution of sexes even among young animals. This isn't necessarily hard-wired into the species; it can change depending on the environment. At some times, mothers consistently produce more sons, and at other times, more daughters - indicating that they have some sort of "choice" in the matter.

There are several reasons why this might be useful, but, for the moment, lets just look at two of the more popular ones. The Trivers-Willard hypothesis applies to species where males normally mate with several females, and states that healthy mothers should tend to have more sons than unhealthy ones. The assumption is that less fit mothers will also tend to have less fit offspring, and that they want to have as many grandchildren as possible. For female offspring that's not much of an issue; they're going to mate with somebody at some point anyway.

But for males offspring, it makes quite a difference - if male offspring aren't fit, they won't get the chance to mate, because the bigger males grab all the females first. So, if you're healthy, have lots of sons, because they will give you lots of grandchildren, but if you're not so fit, concentrate on daughters, because your sons won't have much luck. Its worth noting that 'healthy' in this context, doesn't necessarily refer to disease (although it could), but to anything that affects the chances of males mating - genes for larger antlers, perhaps, or for a more sexually dominant attitude.

The "local resource competition" hypothesis, on the other hand, suggests that mothers living in areas of dense population should have more male offspring than those living elsewhere. This is because, in most mammal species, males tend to wander off in search of females when they reach adulthood, while the females stay around in the area they were born - and, as a result, ensure they don't end up mating with their brothers. (There are a few species where the opposite happens, in which case this theory predicts that they will want female offspring when the population is high). The reason for this is that there is only so much food to go round - if you have lots of daughters, they will hang around and leave less food for you. If the population is high and food is scarce, you want sons that will wander off and find less inhabited places where they aren't pestering you, but, if not, you can afford lots of daughters.

There's no particular reason, of course, why both of these explanations can't be true at the same time, which can make it very difficult to tell what's going on in a particular species.

But all this raises the question of exactly how you ensure you have more children of one sex than the other. Perhaps the most obvious reason why you wouldn't expect this in mammals is that whether your child is male or female depends on their genetics. Males have the XY chromosome pattern, and females the XX, and the two should be produced in reasonably equal numbers. (It's worth noting, incidentally, that the XX/XY system is a peculiarly mammalian thing and isn't true, for example, of reptiles). There is some evidence that rhinoceroses, among others, might be able to influence the survival of embryos in the womb, although the details are not fully clear.

Another problem is that, compared with other animals, mammals spend a lot of time looking after their offspring. Even by the time you've finished being pregnant, and discovered that your offspring isn't the sex you wanted, you've already put a lot of biological resources into raising them, and its a bit of a waste to abandon them now. On the other hand, mammals do at least have the option that they could do that if they really wanted, by giving the offspring less milk. That won't work for mammals that give birth in litters, but it might for those that only have one at a time. In fact, a study conducted on tammar wallabies last year did seem to demonstrate this. The researchers swapped baby wallabies between different mothers, and found that mothers who had given birth to sons did a better job of raising their foster children, even when the foster children were (unbeknownst to the wallaby) female.

Indeed, it has recently been suggested that marsupials are exactly where we should look for sex selection in mammals. Although the authors give a number of reasons for this, perhaps the most apparent is that pregnancy is really very brief in marsupials. That means that, compared with placental mammals, putting less effort into raising offspring after birth wouldn't be quite such a waste of energy. The authors point out that strong biases, mostly in favour of males, do occur in many marsupial species, to the extent that this can be a nuisance for anyone trying to breed them in captivity. A particularly interesting study from 2009, supporting the Trivers-Willard hypothesis, showed that Tasmanian devils infected with a nasty face-eating disease had more daughters than those that were healthy.

Most mammal species do seem to produce roughly equal numbers of sons and daughters, as humans do. But with those that don't, being able to understand how they manage it could have important implications for preserving endangered populations.

Sunday, 6 February 2011

A species, you may have been told, is a group of animals that can breed with each other, but cannot breed with other species to produce fertile offspring. So wolves and domestic dogs are the same species, because they can interbreed, while horses and donkeys are not, because mules are infertile. There are a lot of problems with this idea, because it turns out that nature isn't really all that neat and tidy. For example, polar bears are quite clearly not the same thing as grizzly bears, but the two can produce fertile offspring - something that is happening increasingly as the polar ice caps melt and the natural habitat of the polar bears shrinks.

Its also not at all obvious how you're supposed to apply this definition to animals that don't reproduce sexually (though that's obviously not a problem with mammals). And what about those cases where two species have separated so recently in the evolutionary past that some individuals can crossbreed, but others can't? The classic example here are the herring gulls and lesser black-backed gulls, where the Siberian populations of these two species can interbreed, but the British ones can't. It really does get messy very quickly.

Even if it wasn't for all this, if we really did use the fertile interbreeding definition, how useful would it be? Well, we would have to accept that polar bears are really just odd-looking brown bears, for a start. But more than that, what about other species? Are we really going to try and cross-breed every plausible combination to see what happens? It can be hard enough to get some animals to breed in captivity at the best of times, making the whole idea completely impractical.

Inevitably, this means you sometimes have to end up applying some common sense, and describing something as a separate species largely because it's useful to do so. So polar bears, for example, are considered a species, because they are different in so many ways from brown bears. At the opposite end of the scale, there are many species that don't generally interbreed, but are so closely related that they look almost exactly the same, making it really hard to tell where one species starts and another ends. A lot of these are mice of various kinds, but it's also true of dogs.

The wolf (Canis lupus) is a very widespread species. Before humans drove it off from many parts of its range, it was found throughout Europe, and in most parts of Asia and North America. Human intervention brought them to Australia, in the form of dingos, and the domestic dog is also considered a subspecies of wolf. They once used to inhabit the southern coast of the Mediterranean, but now wild wolves are completely absent from Africa. Or are they?

The closest living relative of the wolf is the golden jackal (Canis aureus). Golden jackals are found in the Balkans, and across Arabia and southern Asia, but they are also found across northern and eastern Africa as far south as Nigeria. It would seem, therefore, that the golden jackal is more suited to these hot climates than its more northerly cousin, the wolf.

Like the wolves, golden jackals are a varied bunch, with several different subspecies recognised. Indeed, one subspecies, found in Egypt, is quite hard to tell apart from a wolf - were it not for the fact that, officially, there are no wolves in Egypt. There had been some dispute in the past as to the exact status of these Egyptian animals, but, more recently, some very wolf-like jackals were also spotted much further south, in Ethiopia. Were they really jackals, as the textbooks said they had to be, or were they something else?

You can't just capture them, breed them with wolves, and see what happens. Not least because we know that wolves can breed with coyotes without any difficulty, and they're certainly a different species, so fertile offspring produced in captivity wouldn't prove anything either way. To see what they really are, we need to look at their genetics. If two groups of animals regularly crossbreed in the wild, their genetics should be very similar, and we have as good a reason as any we can come up with to say that they're really the same species.

So that's what the researchers did. They collected samples of the animals' dung, sequenced their DNA, and compared it with samples taken from jackals in Egypt, with wolves and golden jackals from a range of locations, and, for good measure, from seven other species of the dog family. (Including foxes - when you're doing this sort of test, you need to include something that's sort-of-similar yet manifestly not the same, just to make sure it's working properly).

Of course, they didn't analyse the entire genome, since modern technology isn't up to making that at all routine. Instead, they looked at a particular gene, cytochrome b, which is used as a standard in these sorts of studies. It's presumably not perfect, but with everyone using the same gene, at least it is possible to compare results with those from other workers, and this particular gene has done a good job so far at figuring which mammals are related to which other mammals.

The results showed, firstly, that these Ethiopian jackals were almost identical, genetically, to the ones seen in Egypt. This alone is significant, since it means that this subspecies is found much further south than previously thought. But it also showed that these animals were more closely related to wolves than to other golden jackals. Indeed, in some respects, they were more similar to wolves from places like Canada and Sweden than they were to wolves from the Himalayas.

So, whatever these animals are, they can't possibly be golden jackals. It appears that they diverged from the main group of wolves after the Himalayan wolves did, which means they have to be wolves, too. All the textbooks say there are no wild wolves in Africa, but we now know that there are - at least in Egypt and Ethiopia, where they have been living alongside golden jackals, and looking so similar that few people could tell them apart.

This is exactly the sort of problem that occurs when you have very similar looking species living side by side. Indeed, this sort of finding crops up quite often with mice, with nobody much noticing (or caring, frankly). But even then, is this animal actually a subspecies of wolf, or something else entirely? Just a few years ago, it was argued that Himalayan wolves were themselves a separate species. If that's right, then the Ethiopian animals might well be a species of their own, too - one we didn't previously know about. It's debatable either way, since they do seem to be much closer to northern wolves genetically than we'd expect a new species to be, yet different enough that a blood test will tell them apart.

Does it matter, though? Do we care whether this is a species or a subspecies? Well, funnily, enough, yes it does make quite a difference. Wolves are not an endangered species (and neither are golden jackals), so if these animals are a special sort of wolf, they will get relatively little protection from the various international conventions about such things. But if they're a species, then there's pretty good reason to suppose they might be endangered - there certainly aren't very many of them, and the local farmers do have a habit of killing them to protect their livestock. That's not a problem (from a species conservation perspective) if they're also found elsewhere in the world, but if this is the only place the species is found, then they can get protection under international law.

Considering that we can't come up with a good definition of what a species is, it makes a heck of a lot of difference whether a group of animals is one or not.